Plex not seeing folders on new Volume & new expansion unit

[tom80H]

That sounds very much like a permission issue.
Make sure the system user running your Plex Media Server on the Syno has access to the volumes on the expansion unit.

Synology FAQ - Questions, Answers, and a few How-To's

FAQ 19: Setting permissions to allow Plex access to media files Before beginning, it’s important to remember one key point; Plex doesn’t have any special privileges on Synology except for being able to access the Video Transcoding acceleration hardware. It cannot read any of your files unless you expressly grant permission. This is a fundamental principle of Linux (which Synology uses for DSM) Because of how Synology grants permission, access to files is broken into two different …

You don’t need to create a separate server data directory or library folder. This is not about settings inside Plex.